1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and, more particularly, a method for manufacturing a semiconductor device having predetermined semiconductor elements on an insulating substrate.
2. Description of the Prior Art
The conventional method for manufacturing a semiconductor device having predetermined semiconductor elements on the insulating substrate or a silicon-on-sapphire device (which will be hereinafter referred to as SOS device) in which silicon semiconductor elements are formed on the sapphire substrate includes a process of depositing a silicon single crystal layer on the sapphire substrate, a process of transforming a desired region of silicon single crystal layer to an amorphous construction, and a process of heating the silicon single crystal layer of amorphous structure to 900.degree. C. or 1000.degree. C. and regrowing it. After the completion of these processes, predetermined elements or MOS transistors, for example, are formed in the silicon single crystal layer.
However, the hole mobility of SOS thus formed according to the conventional method is not changed, but the electron mobility thereof is reduced. The field-effect mobility .mu..sub.fe of N-channel MOS transistor formed on (100) plane of P-type bulk Si having an acceptor concentration of about 5.times.10.sup.16 cm.sup.-3, for example, ranges from 900 cm.sup.2 /V.multidot.sec to 1,000 cm.sup.2 /V.multidot.sec, while that of N-channel MOS transistor formed on the SOS including a silicon single crystal layer whose thickness ranges from 0.7 .mu.m to 1 .mu.m is only about 600 cm.sup.2 /V.multidot.sec.
To explain the reason why the above-mentioned drawback is caused, sapphire employed as the insulating substrate has a thermal expansion coefficient two time larger than that of silicon, and when the SOS is returned to room temperature after the finish of its growth process, the sapphire substrate has more of a contraction than the silicon layer. Therefore, the silicon layer is effected by compressive stress acting toward the principal plane thereof when the sapphire substrate contracts, thus causing compressive strain. The lower end of conduction band in the energy band structure of silicon is present in a direction of the main axis of the wave-vector space i.e. of [100] axis, and the equal-energy plane of electrons adjacent to this region is an ellipsoid of revolution taking the main axis direction as its longitudinal axis. Therefore, when under normal condition or no compressive stress is applied, no anisotropy appears in the electron mobility. However, when compressive strain is caused because of contraction of sapphire substrate in the principal plane or XY plane of silicon layer, the energy of the ellipsoid of revolution in the K.sub.z direction i.e. equal-energy plane becomes higher than those of the equal-energy planes in K.sub.x and K.sub.y directions. Therefore, electrons in the K.sub.z valley move to the K.sub.x and K.sub.y valleys, so that the electron mobility in the principal plane of silicon layer depends on increased electrons in K.sub.x and K.sub.y valleys, which have comparatively large mass, and is thus reduced.